1. Field of the Invention
This invention generally relates to the field of communication systems and more particularly to a multiple transmitter wide area communication system requiring fewer system transmitters than conventional communication systems.
2. Description of the Prior Art
Multiple transmitter communication systems are widely used. A typical prior art multiple transmitter communication system is shown in the diagram of FIG. 1. Such a system, which may be used for such purposes as radio paging, includes a number of transmitters located to provide wide area coverage to a plurality of remote paging receivers, or pagers. The coverage area provided by such prior art systems was determined by a number of factors, such as transmitter power output, transmitter antenna height, pager antenna sensitivity, type of terrain, and size and density of buildings in the coverage area. Messages to be transmitted to the remote pagers were generally simulcast throughout the system to provide the highest system through-put, while maintaining reliable message delivery throughout the system. As shown in FIG. 1, the typical system included a central station 12 having a coverage area of radius r, often surrounded wholly or partially by a plurality of remote stations 14 each having a coverage area of r'. Such prior art systems were designed to provide at least a 90% probability of signal reception within a building, and in excess of 98% probability of signal reception on the street at the zone, or cell, boundary of radius r for the central station 12 and at the cell boundary of radius r' for the remote stations 14. The area covered by the central station 12 was generally greater than covered by the remote stations 14, due to either the central station 12 having a greater transmitter power output or greater antenna height as compared to the remote stations 14. A typical paging system configured as shown in FIG. 1, provided a 90% probability of signal reception on-the-street, and approximately a 60% probability of signal reception in-building, over an area having a radius R for transmissions originated from the central station. In the typical paging system, radius R corresponds to approximately a twenty mile radius, for a total system coverage somewhat in excess of twelve hundred square miles. When information was simulcast transmitted from the central station and the remote stations, the probability of signal reception improved to in excess of 98% on-the-street and 90% in-building coverage over the area encompassed within radius R.
Paging systems, such as described in FIG. 1, have provided excellent results for a broad range of pagers operating on a wide range of frequencies, and utilizing a variety of signaling formats, such as the well known POCSAG and Golay Sequential Code (GSC) signaling formats. Such systems, however, have rapidly become limited in the ability to deliver information, as the number of subscribers operating in the system becomes large, such as in excess of several thousand subscribers, largely due to delays in queuing the messages and the corresponding reduction in system through-put. The problems have been compounded further as the amount of information to be delivered to each subscriber increases. With memory costs decreasing, pagers providing thousands of characters for message storage are realizing an increased demand. The increased storage capacity of such pagers has made such developments as information services and digitized voice transmission practical. Such increased demand for data has resulted in a need to develop pagers and systems capable of handling the increased data requirements. For the prior art system of FIG. 1, the typical method for providing increased message through-put required by the increased information input, was by increasing the bit rate at which the messages are delivered. While this method has somewhat proven to improve message through-put- while minimizing the number of, and complexity of, transmitters provided in the system, there is a limit to which such bit rate increase can occur, before system problems are encountered in the prior art system of FIG. 1. This limit lies between 2400 and 4800 bits per second, at which point it becomes increasingly impractical to design systems which can provide adequate simulcast transmissions. Historically, the next step to resolving the data rate transmission problem has been by adding additional channels to the system at both the central station and each remote station. Prior art systems utilized an equal number of transmitters at both the central station and the remote stations, and when additional capacity was required, additional transmitters operating on new channels were provided, assuming such frequency spectrum allocation was available, at the central station and at each remote station. Such a solution added both a significant cost and complexity to the system. Furthermore, such systems, in actuality provided excess capacity which was not adequately being utilized, in order to guarantee message through-put and message delivery.